Method and plant for separating polymeric materials

Abstract

A method for recycling polymeric materials, including a mechanical separation step (B), a first density separation step (C), an intensive cleaning step (E), and a second density separation step (G, V) is described. Additional separation steps, such as a third density separation step (M, W), and optionally a fourth density separation step (G a , G b ). Further less essential steps such as wetting (A), grinding (D), air flow separation (K), drying (I), electrostatic sorting (Z) and optional sorting (Z a ) steps may also be carried out to achieve optimized and refined separation. Polymeric materials from refuse or crushed motor vehicles may thus be separated, and polyethylene, polypropylene, polystyrene and acrylonitrile-butadiene-styrene resins may be recycled.

1. A method for treating waste materials to separate polymer materials from other materials contained in the waste materials, the method comprising:
 (a) grinding the waste materials,  
 (b) treating the ground waste materials by a first density separation to separate first treated waste materials having a density less than a first density from second treated waste materials having a density greater than the first density,  
 (c) treating the first treated waste materials by mechanical separation to separate third treated waste materials having a selected form factor from fourth treated waste materials which do not have the selected form factor,  
 (d) grinding the third treated waste materials,  
 (e) cleaning the ground third treated waste materials, and  
 (f) treating the cleaned, ground third treated waste materials by a second density separation to separate fifth treated waste materials having a density less than a second density from sixth waste materials having a density greater than the second density, wherein the fifth treated waste materials comprise polymer materials.  
 
     
     
       2. The method according to  claim 1 , wherein the step of mechanical separation is performed in a screening device comprising a rotating drum with a calibrated grid. 
     
     
       3. The method according to  claim 1 , wherein the step of cleaning the third treated waste materials comprises:
 (f) providing at least one tank containing a liquid cleaning medium;  
 (g) placing the third treated waste materials in the tank such that the tank contains from about 50% to about 70% polymer materials and from about 30% to about 50% liquid medium by volume; and  
 (h) stirring the contents of the tank.  
 
     
     
       4. The method according to  claim 3 , wherein the step of cleaning the third treated waste materials further comprises:
 (i) heating the third treated waste materials and the liquid cleaning medium in the tank to a temperature between room temperature and the boiling temperature of the liquid medium.  
 
     
     
       5. The method according to  claim 3 , wherein the liquid medium comprises from about 50% to about 100% by weight of water, from about 0% to about 20% by weight of cleaning agent, from about 0% to about 30% by weight of an abrasive agent, and from abut 0% to about 5% by weight of a wetting agent. 
     
     
       6. The method according to  claim 5 , wherein the cleaning agent is selected from the group consisting of sodium hydroxide, potassium hydroxide, sodium carbonate, soap and mixtures thereof. 
     
     
       7. The method according to  claim 5 , wherein the abrasive agent is selected from the group consisting of calcium carbonate powder, talc, silica, alumina, and mixtures thereof. 
     
     
       8. The method according to  claim 1 , wherein the first and second density separations are performed in a liquid medium having a controlled density such that material having a density less than that of the liquid medium is separated from material having a density greater than that of the liquid medium. 
     
     
       9. The method according to  claim 8 , wherein the first treated waste materials have a density of less than or equal to 1 g/cm 3  and the second treated waste materials have a density of greater than 1 g/cm 3 . 
     
     
       10. The method according to  claim 9 , wherein the fifth treated waste materials have a density of less than or equal to 1 g/cm 3  and the sixth treated waste materials have a density of greater than 1 g/cm 3 . 
     
     
       11. The method according to  claim 10 , wherein the liquid medium comprises water. 
     
     
       12. The method according to  claim 10 , further comprising the step of:
 (g) treating the fifth treated waste materials by a third density separation in a liquid medium having a controlled density to separate seventh treated waste materials having a density less than a third density from eighth treated materials having a density greater than the third density.  
 
     
     
       13. The method according to  claim 12 , wherein the density of the liquid medium of the third density separation is adjusted to about 0.90 g/cm 3  to about 0.92 g/cm 3  in order to separate propylene polymers having a density of less than about 0.90 g/cm 3  from ethylene polymers having a density greater than about 0.92 g/cm 3 . 
     
     
       14. The method according to  claim 8 , wherein the liquid medium comprises water, wetting agents and at least one of clays and salts such that the density of the liquid medium is greater than about 1 g/cm 3 . 
     
     
       15. The method according to  claim 8 , wherein the liquid medium comprises water and at least one of methanol, ethanol and a wetting agent, such that the density of the liquid medium is less than 1 g/cm 3 . 
     
     
       16. The method according to  claim 1 , further comprising the step of wetting the first treated waste materials with an alkaline solution before the step of treating the ground waste by mechanical separation. 
     
     
       17. The method according to  claim 1 , further comprising:
 (h) removing liquid from the fifth treated waste materials using a centrifuge,  
 (i) drying the centrifuged fifth treated waste materials, and  
 (j) storing the dried fifth treated waste materials in silos.  
 
     
     
       18. The method according to  claim 1 , further comprising:
 (h) removing liquid from the fifth treated waste materials using a centrifuge,  
 (i) drying the centrifuged fifth treated waste materials,  
 (j) providing air flow through the dried fifth treated waste materials to remove light materials from the dried fifth treated waste materials, and  
 (k) after step (j), storing the dried fifth treated waste materials in at least one silo.  
 
     
     
       19. The method according to  claim 1 , further comprising:
 (h) removing liquid from the fifth treated waste materials using a centrifuge,  
 (i) drying the centrifuged fifth treated waste materials and providing air flow through the dried fifth treated waste materials to remove light materials from the fifth treated waste materials,  
 (j) after step (i), treating the dried fifth treated waste materials by a third density separation using a mechanical density separation device to separate seventh treated waste materials having a density less than a third density from eighth treated waste materials having a density greater than the third density, and  
 (k) storing the seventh treated waste materials and the eighth treated waste materials in silos.  
 
     
     
       20. The method according to  claim 1 , further comprising
 (g) removing liquid from the fifth treated waste materials using a centrifuge;  
 (h) drying the centrifuged fifth treated waste materials;  
 (i) homogenizing polymer materials contained in the fifth treated waste materials in a first homogenizing step;  
 (j) extruding the homogenized polymer materials into pellets;  
 (k) separating water from the polymer pellets using a centrifuge;  
 (l) grading the polymer pellets based upon size;  
 (m) homogenizing the graded polymer pellets in a second homogenizing step; and  
 (n) bagging the polymer pellets.  
 
     
     
       21. A method for treating waste materials to separate polymer materials from other materials contained in the waste materials, comprising the following steps:
 (a) grinding the waste materials,  
 (b) treating the ground waste materials by a first density separation to separate first treated waste materials having a density greater than a first density from second treated waste materials having a density less than the first density,  
 (c) treating the first treated waste materials by mechanical separation to separate third treated waste materials having a selected form factor from fourth treated waste materials which do not have the selected form factor,  
 (d) grinding the third treated waste materials,  
 (e) cleaning the ground third treated waste materials, and  
 (f) treating the cleaned, ground third treated waste materials by a second density separation to separate fifth treated waste materials having a density less than a second density from sixth treated waste materials having a density greater than the second density, wherein the fifth treated waste materials comprise PS and the sixth treated waste materials comprise ABS.  
 
     
     
       22. The method according to  claim 21 , wherein the step of mechanical separation is performed in a screening device comprising a rotating drum with a calibrated grid. 
     
     
       23. The method according to  claim 21 , wherein the step of cleaning the third treated waste materials comprises:
 (g) providing at least one tank containing a liquid cleaning medium;  
 (h) placing the third treated waste materials in the tank such that the tank contains from about 50% to about 70% by volume third treated waste materials and from about 30% to about 50% by volume liquid cleaning medium; and  
 (i) stirring the contents of the tank.  
 
     
     
       24. The method according to  claim 23 , wherein the step of cleaning the third treated waste materials further comprises:
 (j) heating the third treated waste materials and the liquid cleaning medium to a temperature between room temperature and the boiling point of the liquid medium.  
 
     
     
       25. The method according to  claim 23 , wherein the liquid cleaning medium comprises from about 50% to about 100% by weight of water, from about 0% to about 20% by weight of cleaning agent, from about 0% to about 30% by weight of an abrasive agent, and from abut 0% to about 5% by weight of a wetting agent. 
     
     
       26. The method according to  claim 25 , wherein the cleaning agent is selected from the group consisting of sodium hydroxide, potassium hydroxide, sodium carbonate, soap and mixtures thereof. 
     
     
       27. The method according to  claim 25 , wherein the abrasive agent is selected from the group consisting of calcium carbonate powder, talc, silica, alumina, and mixtures thereof. 
     
     
       28. The method according to  claim 21 , wherein the first and second density separations are performed in a liquid medium having a controlled density such that material having a density less than that of the liquid medium is separated from material having a density greater than that of the liquid medium. 
     
     
       29. The method according to  claim 28 , wherein the first selected density is about 1 g/cm 3 , and the second selected density is about 1.05 g/cm 3 . 
     
     
       30. The method of  claim 29 , further comprising:
 (g) treating the sixth treated waste materials by a third density separation using a mechanical density separation device to separate seventh treated waste materials having a density less than a third density from eighth treated waste materials having a density greater than the third density, wherein the eighth treated waste materials comprise ABS.  
 
     
     
       31. The method of  claim 30 , wherein the mechanical density separation device is a densimetric shaking table. 
     
     
       32. The method of  claim 31 , further treating the eighth treated waste materials by electrostatic sorting to remove impurities from the ABS in the eighth treated waste materials. 
     
     
       33. The method according to  claim 29 , further comprising:
 (g) treating the sixth treated waste materials by a third density separation in a liquid medium having a controlled density to separate seventh treated waste materials having a density of less than about 1.1 g/cm 3  from eighth treated waste materials having a density greater than about 1.1 g/cm 3 ; and  
 (h) treating the seventh treated waste materials by a fourth density separation in a liquid medium having a controlled density to separate ninth treated waste materials having a density less than 1.07 g/cm 3  from tenth treated waste materials having a density greater than 1.07 g/cm 3 , wherein the tenth treated waste materials comprise ABS.  
 
     
     
       34. The method of  claim 33 , further comprising:
 (g) treating the tenth treated waste materials by a fifth density separation using a mechanical density separation device to separate eleventh treated waste materials having a density less than a third density from twelfth treated waste materials having a density greater than the third density, wherein the twelfth treated waste materials comprise ABS.  
 
     
     
       35. The method of  claim 34 , wherein the mechanical density separation device is a densimetric shaking table. 
     
     
       36. The method of  claim 35 , further treating the twelfth treated waste materials by electrostatic sorting to remove impurities from the ABS in the twelfth treated waste materials. 
     
     
       37. The method of  claim 21 , further comprising:
 (g) treating the fifth treated waste materials by a third density separation using a mechanical density separation device to separate sixth treated waste materials having a density less than a third density from seventh treated waste materials having a density greater than the third density, wherein the seventh treated waste materials comprise PS.  
 
     
     
       38. The method of  claim 37 , wherein the mechanical density separation device is a densimetric shaking table. 
     
     
       39. The method of  claim 38 , further comprising treating the seventh treated waste materials by optical sorting to remove filled PE and filled PP from the PS in the eighth treated waste materials. 
     
     
       40. The method according to  claim 21 , further comprising:
 (g) treating the fifth treated waste materials by a third density separation in a liquid medium with a controlled density to separate sixth treated waste materials having a density less than about 1.05 g/cm 3  from seventh treated waste materials having a density greater than about 1.05 g/cm 3 , wherein the sixth treated waste materials comprise PS.  
 
     
     
       41. The method of  claim 40 , wherein the mechanical density separation device is a densimetric shaking table. 
     
     
       42. The method of  claim 41 , further treating the sixth treated waste materials by optical sorting to remove filled PE and filled PP from the PS in the sixth treated waste materials.